1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device and a method of fabricating the same that reduces a gray inversion phenomenon.
2. Description of the Related Art
A liquid crystal display LCD device displays a picture by controlling an electric field applied to a liquid crystal cell to modulate the light incident to the liquid crystal cell. Depending on the direction of the applied electric field driving the liquid crystal, a liquid crystal display device is broadly classified as a vertically applied electric field type or a horizontally applied electric field type.
In the vertically applied electric field type of LCD device, a pixel electrode and a common electrode are formed in an upper substrate and a lower substrate facing each other in a vertical direction. A vertical electric field is generated by applying a voltage to the pixel and common electrodes. Twisted nematic (hereinafter, referred to as “TN”) mode LCD is an example of the vertical electric field type LCD. Most of the current liquid crystal display devices use the TN mode.
FIG. 1A is a schematic view of a liquid crystal display panel of a related art twisted nematic mode LCD in an off-state. FIG. 1B is a schematic view of a liquid crystal display panel of the related art twisted nematic mode LCD in an on-state. Referring to FIGS. 1A and 1B, a TN mode LCD panel 10 includes an upper substrate 7 and a lower substrate 3. A liquid crystal layer 5 is provided between the upper substrate 7 and the lower substrate 3. A pixel electrode (not shown) and a common electrode (not shown) are formed in the upper substrate 7 and the lower substrate 3, respectively. For the sake of explanation, FIGS. 1A and 1B show the shapes of the liquid crystal molecules when seen from a three o'clock direction.
An upper polarizer 9 is adhered to a light exit surface of the upper substrate 7. A lower polarizer 11 is adhered to a light incidence surface of the lower substrate 3. The upper polarizer 9 has a light transmission axis of a first direction. The lower polarizer 11 has a light transmission axis perpendicularly to the light transmission axis of the upper polarizer 9.
The TN mode LCD can be operated in a normally white mode. As shown in FIG. 1A, when a voltage is not applied between the pixel electrode in the upper substrate 7 and the common electrode in the lower substrate 3, the TN mode LCD is in an off-state. The local optical axes (director) of the liquid crystal molecules are continuously twisted by 90° between the upper substrate 7 and the lower substrate 3 in the off-state. In the off-state, the polarization state of a linearly polarized light which is incident through the lower polarizer 11 changes while passing through the liquid crystal layer 5 to allow transmission of the polarized light through the upper polarizer 9.
In contrast, when a voltage is applied between the pixel electrode in the upper substrate 7 and the common electrode in the lower substrate 3, the TN mode LCD is in an on-state and an electric field is generated in the liquid crystal. As shown in FIG. B, the optical axis of the middle part of the liquid crystal layer 5 becomes parallel to the generated electric field. The variation in applied voltage between the pixel electrode (not shown) and the common electrode (not shown) from the off-state to the on-state releases the twisted structure in the TN mode LCD. In the on-state, the linearly polarized light incident through the lower polarizer 11 maintains its polarization state while passing through the liquid crystal layer 5 to block transmission of the polarized light through the upper polarizer 9.
The vertical electric field type LCD can provide a relatively broad aperture ratio. The TN mode also has a high transmissivity, and is relatively easy to produce. However, the vertical electric field type LCD typically provides a narrow viewing angle. Moreover, the vertical electric field type LCD can suffer from a gray inversion phenomenon appearing at a lower viewing angle, for example, at lower part of the liquid crystal display panel.
FIG. 2 is a graphical view of experimental data representing changes in gray level in accordance with a viewing angle. As shown in FIG. 2, the gray inversion phenomenon causes a dark gray level to appear brighter than a bright gray level at a lower viewing angle. FIG. 3 is a photograph which compares a gray inversion screen and a normal screen. As shown in FIG. 3, the quality of the picture based on the gray inversion is quite distorted from the normal picture. A main cause for the generation of the gray inversion is a variation of refractive index in accordance with the viewing angle.
FIG. 4A is a schematic view of a variation in birefringence depending on viewing angles when the related art twisted nematic mode LCD panel is in the off-state. The birefringence of a liquid crystal layer having a thickness “d” and a refractive index “Δn” is defined as “dΔn.” As shown in FIG. 4A, a TN mode LCD panel 10 has a birefringence dΔn1 in a first viewing direction corresponding to a lower viewing angle in which a user faces the screen from the lower part of the panel 10. The TN mode LCD panel 10 has a birefringence dΔn2 in a second viewing direction corresponding to an upper viewing angle in which the user faces the screen from the upper part of the panel. The TN mode LCD panel 10 has a birefringence dΔn3 corresponding to a third viewing direction in which the user faces the screen from the front part of the panel. When the TN mode liquid crystal display panel 10 is in the off-state, there is almost no change in the birefringence dΔn1 in the first viewing direction, the birefringence dΔn2 in the second viewing direction, and the birefringence dΔn3 in the third viewing direction.
FIG. 4B is a schematic view of a variation in birefringence depending on viewing angles when the related art twisted nematic mode LCD panel is in the on-state. As shown in FIG. 4B, when the TN mode LCD panel is in the on-state, an average director (A) of the liquid crystal is tilted around an axis at a crossing of the first and third viewing directions. Thus, the birefringence dΔn of the light passing through the liquid crystal changes in accordance with the viewing angle. Specifically, the birefringence in the first, second and third viewing directions follows the relation of dΔn1<dΔn2<dΔn3. As shown in FIG. 4B, the short axis of the liquid crystal is observed in the first viewing direction, and longer axes are observed in the second and third viewing directions. Thus, the desired gray level is achieved in the first viewing direction, but brighter or dimmer gray levels appear in the second and third viewing directions as the dΔn value become higher, in accordance with the P1 area in FIG. 2.
As shown in FIG. 4B, when the user sees the screen from a fourth direction at a lower viewing angle than the first viewing direction, the birefringence dΔn4 in the fourth direction becomes higher than the birefringence dΔn1 in the first viewing direction. Thus, the gray level in the fourth direction appears to be brighter than the gray level in the first viewing direction, in accordance with the P2 area in FIG. 2.
FIG. 5 is a schematic diagram of a gray inversion generation on the related art twisted nematic mode LCD panel. As shown in FIG. 5, a zero point where the refractive index Δn of the average director of the liquid crystal becomes theoretically “0” is a peak. The birefringence dΔn of a dark gray level is higher than the birefringence dΔn of a bright gray level at a viewing angle lower than the zero point and corresponding to a lower part of the panel. Such a phenomenon appears on the screen as a gray inversion.